Chemical mechanical planarization, also known as CMP is a technique used to planarize the top surface of an in-process semiconductor wafer or other substrates in preparation of subsequent steps or for selectively removing material according to its position. Both chemical and mechanical actions are generally involved.
CMP is commonly used in microelectronic integrated circuit (IC) fabrication. ICs are multilayered structures comprised of dielectric and conductive layers that are also patterned laterally in order to isolate different devices and functions. Each layer is deposited sequentially and then polished/removed by CMP to a desired depth prior to the deposition of the next layer in the sequence. Once a layer has been processed by CMP, it can undergo a post CMP cleaning operation that typically includes exposure (e.g., for a few minutes) at a cleaning station during which ammonium hydroxide solutions flow over a wafer surface with gentle polymer brush scrubbing action followed by forced air or infrared heat drying. Typically, the cleaning operations are followed by metrology steps to measure film thickness by either optical methods (e.g., ellipsometry) for oxide layer or by electrical impedance techniques for metallic layers (e.g., four point probe measurements).
CMP can be used to planarize a surface to nanometer and sometimes angstrom levels of smoothness in order to maximize the fidelity and quality of the pattern deposited in the next layer. CMP is necessary after nearly every material deposition step (eg. conductive metal, metal barrier, or oxide insulator layers as examples).
Removal rate is a key feature for CMP as it dictates the speed at which a given CMP step can be conducted. Also important are considerations related to workpiece defects such as those caused by scratching, dishing of metal features, oxide erosion and so forth. Since defective workpieces often need to be re-processed or discarded, their occurrence has a major impact on the costs and efficiency of the overall manufacturing process.
Typical CMP operations involve the cyclic motion of a polishing pad and a workpiece in the presence of a slurry that has abrasive and/or corrosive properties. In semiconductor manufacturing, for instance, a wafer is mounted on carrier and pressed down on a rotating platen holding a compliant polishing pad. Slurry is dispensed at the interface between wafer and pad and wafer material is removed by the combined actions of the chemical slurry and abrasive properties of the pad. The rotation of the head, coupled with the motion of the pad and its topography serves to introduce the wafer to a continuous flow of fresh slurry.
Generally, CMP polishing pads are considerably larger than the workpiece and are fabricated from polymeric materials that can include certain features, such as, for example, micro-texture for retaining the slurry on the pad. Polishing pad properties can contribute to within die (WID) thickness uniformity and within wafer (WIW) planarization uniformity. As described in U.S. Pat. No. 6,572,439, issued on Jun. 3, 2003 to Drill et al., for many CMP processes using a harder, less compressible polishing pad can lead to an increase in WID thickness uniformity but a reduction in WIW planarization uniformity, whereas a softer, more compressible pad can have the opposite effects. Such inconsistencies can be further exacerbated on larger (e.g., 300 to 450 mm) wafer scales.
With use, the working surface of the pad can become eroded. Polishing debris can get trapped in the surface micro-channels, clogging them. A conditioning or “dressing” operation can be performed (with a conditioning tool, often a diamond containing abrasive conditioning pad) to remove the glazed layer and expose a fresh polishing surface for contacting the workpiece.
CMP slurries generally contain abrasive particles, often in conjunction with other materials, in an aqueous medium. The type and properties of the abrasive can be selected by taking into account the material being planarized, desired surface finish (expressed, for example, in terms “out of flatness” or as Ra values) and other criteria. Exemplary abrasive particles that can be utilized include but are not limited to silica, alumina, ceria, zirconia, silicon carbide, and others. The abrasive particles can have characteristics that enhance slurry performance during CMP (e.g., with respect to removal rates, reliability, reproducibility or number of defects). U.S. Pat. No. 7,037,451, issued to Hampden-Smith et al. on May 2, 2006 and incorporated herein by reference in its entirety, for example, describes CMP slurries that contain abrasive particles that have a small particle size, narrow size distribution, a spherical morphology and are substantially unagglomerated.
Typical silica-containing CMP slurries use fumed silica of low surface area (90 m2/g), such as Cab-O-Sil™ L-90 fumed silica (nominal specific surface area of 90 m2/g), in concentrations ranging from 5 to 15 wt % or colloidal silica (also referred to herein as sol gel silica) of similar surface areas and loadings. Often higher loadings of colloidal silica slurries are required to achieve comparable CMP material removal rates. For example, in oxide or interlayer dielectric (ILD) CMP polishing, the typical loading is 10-12% wt for fumed silica in the slurry. By comparison, typical colloidal silica loadings for comparable ILD polishing slurries would be 25-30% wt (e.g. for example, Klebesol 1501 silica slurry).
The concentration and size of the particle largely dictates the removal rate, particularly for ILD CMP steps. Lower specific surface area particles (e.g. larger diameter particles) provide an advantage over other smaller particles (higher surface area) in terms of removal rate. However, defectivity (often expressed as the number of scratches) also increases with particle size and can reduce device yield by ultimately causing chip failures that are not detected until fabrication is completed. While decreasing the loading of particles can reduce defectivity, it also reduces removal rate, since removal rate scales with particle loading. Rate accelerating chemistry packages, sometimes called accelerators, added to the slurry can help augment removal rate, allowing the usage of smaller particles and reduced loadings to match removal rate and reduce defectivity. Smaller particles, however, are also more difficult to detect during post CMP metrology steps (e.g. defectivity measurements), and they are more difficult to remove by cleaning operations that follow CMP. These residual particles pose the threat of greater defectivity and lost yield, as subsequently deposited layers in the IC architecture trap the defect residual particle, which can ultimately cause performance issues in the final product. The size of the primary particle in a fumed aggregate or colloidal particle can also influence removal rate.